1. Field of the Invention
The present invention relates to a semiconductor integrated circuit having an oscillation circuit and, more particularly, to a horizontal oscillation circuit which can change the frequency of a horizontal sync signal over a wide range and is applied to a monitor or a television receiver.
2. Description of the Related Art
A PLL (Phase Locked Loop) circuit is used as a horizontal oscillation circuit in a monitor used for, e.g., a computer, or a television receiver. This PLL circuit has a voltage-controlled oscillator (to be referred to as a VCO hereinafter) for generating a clock signal having a frequency n (n is an integer) times higher than the frequency of a horizontal sync signal.
Conventionally, the VCO is only required to generate clock signals of the same frequency. A recently developed monitor called a multi-scanning monitor can switch scanning speeds. The frequency of a horizontal sync signal supplied from a computer to a monitor of this type changes, for example, in the range of 20 kHz to 100 kHz. For this reason, a VCO applied to a horizontal oscillation circuit is required to be capable of oscillating over a wide range of the frequencies of horizontal sync signals.
If one VCO is made to oscillate over a wide frequency range, the VCO requires a high oscillation gain. If a high gain is set, the frequency response speed increases, resulting in a deterioration in jitter characteristics.
In order to solve this problem, a plurality of VCOs with different oscillation frequency ranges may be arranged such that the VCOs are switched to operate in accordance with the frequency of a horizontal sync signal. In this case, however, in addition to a plurality of VCOs, the oscillation circuit requires a frequency/voltage converter for generating a voltage corresponding to the frequency of a horizontal sync signal, resulting in an-undesirable increase in circuit size.